Beam-forming is a well-known technique in the field of wireless communication. It may, for example, be used for improving the signal-to-noise ratio (SNR) for a communication link by steering the transmitted energy in, or collecting the received energy from, a favorable direction. As referred to herein, beam-forming may conceptually be accomplished by any suitable known or future techniques, and applicable details of those techniques will not be elaborated on further.
Determining which beam should preferably be used (e.g. an optimal beam or a sufficiently good beam according to some criteria) in a particular situation and scenario may be challenging, especially if a large number of antenna elements and high beam-forming resolution is applied. In a typical implementation of a system using beam-forming, a receiving device may perform measurements of reference signals sent from a transmitting device and feed back measurement results (e.g. a preferred beam).
Precoding is a kind of beam-forming that supports multi-layer transmission in multi-antenna radio systems, such as multiple-input multiple-output (MIMO) radio systems, and is relevant in relation to, for example, the 3GPP (Third Generation Partnership Project) standard Universal Mobile Telecommunication Standard, Long Term Evolution (UMTS-LTE).
Closed loop MIMO precoding is a MIMO approach where each of the multiple streams are emitted from the transmit antennas (e.g. at a base station or other network node) with independent and appropriate weighting per each antenna such that the throughput is ideally maximized between the transmitting device and the receiving device (e.g. a user equipment (UE) of UMTS-LTE). The precoding weights that should be used are typically calculated based on measurements at the receiving device and communicated to the transmitting device in a similar manner as explained above.
Usually, only a limited number of (predefined) precoding weights are used. The collection of precoding weights is called a codebook. In closed loop MIMO precoding, the codebook is typically known at both the transmitting and the receiving device. Hence, communication of preferred precoding weights by the receiving device may simply comprise indicating the index that the preferred precoding weights have in the codebook. This is typically done by sending a number which is usually called the Precoding Matrix Indicator (PMI).